In recent years, energy savings and shifts to clean energy have advanced. Along with these, a power device market scale has increased.
For manufacturing power devices, a silicon wafer is used, for example. Such a silicon wafer preferably contains almost no impurity including carbon and oxygen. Contamination of the silicon wafer with impurities is, however, inevitable in the manufacturing process thereof. For this reason, it is important to know the concentrations of impurities contained in the silicon wafer.
A silicon single-crystal having desirable characteristics for a power device substrate has a carbon concentration equal to or lower than the lower quantitation limit of secondary ion mass spectrometry (SIMS), which is a known measurement method. Thus, it had been difficult to accurately determine the carbon concentration in such a silicon single-crystal. Under these circumstances, a measurement method using a photoluminescence (PL) method was proposed as a method of measuring a low carbon concentration.
The PL method is a method which includes irradiating a material with excitation light and observing light emitted when the transition of the excited electrons to the ground state occurs.
Jpn. Pat. Appin. KOKAI Publication No. 2013-152977 describes a method of measuring a concentration of an impurity in a semiconductor wafer using the PL method. According to this method, the semiconductor wafer is irradiated with an electron beam to render the impurity contained in the semiconductor wafer luminescence-active, and then the luminescence intensity is measured by the PL method. A ratio of the intensity at 1,280 nm to the intensity at 1,570 nm is obtained from the spectrum. A calibration curve representing the relationship between the luminescence intensity ratio thus obtained and the concentration of the impurity contained in the semiconductor wafer is then determined. Thereafter, the calibration curve is extrapolated to a lower carbon concentration range, and the measurement of the luminescence intensity is performed on a wafer containing an impurity at a low concentration. This luminescence intensity is referred to the calibration curve to obtain the impurity concentration.
Jpn. Pat. Appin. KOKAI Publication No. 2015-101529 describes a carbon concentration measurement method by which the influence of the oxygen concentration on a measurement result is small. According to this method, luminescence intensity ratios obtained using the PL method and concentration division values, which are ratios of carbon concentrations to oxygen concentrations, are obtained for a plurality of silicon single-crystals having different carbon and oxygen concentrations. A calibration curve representing the relationship between the photoluminescence ratios and the concentration division values is determined. Thereafter, the luminescence intensity measurement is performed on a silicon single-crystal specimen, and the measurement results are referred to the calibration curve to obtain the concentration division value. Subsequently, the oxygen concentration of a silicon single-crystal specimen is measured, and the carbon concentration is calculated from the oxygen concentration and the concentration division value.